In amplifiers used for radio communication of mobile telephones and so on, generally two or three compound semiconductor transistors for high frequency amplification are connected in multiple stages. In recent years, hetero bipolar transistors are mainly used as compound semiconductor transistors in consideration of a single positive power supply operation and so on.
When a current driving bipolar transistor is used for an amplifier having an output power of about 1 to 3 W for mobile telephone terminals, a bias circuit is generally used. The bias circuit includes, in addition to a bipolar transistor for high frequency amplification, another bipolar transistor which supplies a bias to the base of the transistor by means of an emitter follower. (see Japanese Patent Laid-Open No. 2003-273660).
Further, methods using a directional coupler and a detector circuit are generally used to detect an output power of an amplifier and control the bias voltage of the amplifier according to the output power.
FIG. 20 shows an example of a conventional high frequency amplifier.
A high frequency input signal is inputted from a high frequency power input terminal 17 to the base of a bipolar transistor 1 for high frequency amplification (hereinafter, referred to as a transistor 1), is outputted from the collector thereof, and is outputted from a high frequency power output terminal 18.
An input matching circuit 9 converts the input impedance of the transistor 1 into 50Ω. An output matching circuit 11 sets an impedance from the output of the transistor 1 so as to obtain a desired power for a load of 50Ω.
A bias is supplied from a voltage source 13 to the collector of the transistor 1 via a collector power supply circuit 12 such as a λ/4 wavelength line. Base bias is supplied from a voltage source 14 to the base of the transistor 1 by means of an emitter follower of a bipolar transistor 2 for bias supply. The bias current of the transistor 1 is adjusted according to the voltage of the voltage source 14 and a resistor 7. A resistor 8 suppresses excessive base current of the transistor 1.
Reference numeral 15 denotes a voltage source for supplying the collector bias of the bipolar transistor 2 for bias supply.
FIG. 21 shows that the output power and temperature dependence of the high frequency amplifier shown in FIG. 20 are improved by using the diode 5 for generating the reference voltage. To be specific, the base electric potential of the bipolar transistor 2 for bias supply is so adjusted by an operational amplifier 3 as to be equal to a voltage generated in the diode 5 for generating the reference voltage. The transistor 2 supplies base bias to the transistor 1 by means of the emitter follower and the diode 5 has been biased by a constant current source 4. The bias current of the transistor 1 is adjusted by the constant current source 4.
FIG. 22 shows an example of a high frequency amplifier having the function of adjusting the bias of a high frequency transistor according to an output power. A high frequency signal is inputted from a high frequency power input terminal 17. An output from a high frequency amplifier 61 passes through a directional coupler 64 and is outputted from a high frequency power output terminal 18. The output is partially inputted to a detector circuit 65 by the directional coupler 64. The output voltage of a control power supply circuit 66 is controlled for a desired characteristic according to the output level of the detector circuit 65. The voltage is supplied to a base or gate voltage supply terminal 62 and a collector or drain voltage supply terminal 63 of the high frequency amplifier 61.
However, in the case of the high frequency amplifier using the conventional bipolar transistor, when the output load of the high frequency amplifier fluctuates, the bipolar transistor for high frequency amplification rapidly increases in collector current and thus a breakdown may occur in the transistor.
Generally, a breakdown is prevented by the following methods:                1) A resistor is provided in the collector or emitter of a high frequency transistor.        2) A resistor is provided in the base of a high frequency transistor.        3) The size of a high frequency transistor is increased to have a higher allowable current.        4) The output power of a high frequency amplifier is detected and a bias voltage is controlled so as to prevent the occurrence of a breakdown in the high frequency amplifier at high output power.        
However, in 1), power added efficiency is seriously reduced by a resistance loss in the high-power amplifier. In 3), power gain and power added efficiency decrease because the high frequency transistor increases in size. Further, a semiconductor chip increases in size.
Although method 2) is most commonly used, a voltage drop in the resistor is proportionate to a base current and thus a resistance value enough to prevent a breakdown in the event of fluctuations in output load degrades a characteristic in an actually used area. Method 4) requires a directional coupler and a detector circuit, so that a characteristic is degraded by a loss in the directional coupler and the detector circuit and the high frequency amplifier increases in size.
As described above, in the conventional art, when the collector current of an amplifier increases due to fluctuations in the output load of the amplifier, a breakdown may occur in a transistor. In order to prevent a breakdown, the high frequency characteristic of the amplifier degrades and the size of the amplifier increases.
It is an object of the present invention to provide a high frequency amplifier which can prevent a breakdown in a transistor without degrading the high frequency characteristic of the high frequency amplifier.